Pronounced retention of stereoconfiguration upon sp3 C H bonds hydroxylation of dimethylcyclohexanes and decahydronaphthalenes with m-CPBA oxidant and a Co-phthalocyanine catalyst

A synthetically useful catalytic system for aliphatic C-H oxidation with a nonheme cobalt complex and m-CPBA

We report herein a synthetically useful catalytic system for aliphatic C-H oxidation with a mononuclear nonheme cobalt(II) complex and m-chloroperbenzoic acid (m-CPBA). Preliminary mechanistic studies suggest that a high-valent cobalt-oxygen species (e.g., cobalt(IV)-oxo or cobalt(III)-oxyl) is the oxidant that effects C-H oxidation via a rate-determining hydrogen atom abstraction (HAA) step.

A Family of Hexacopper Phenylsilsesquioxane/Acetate Complexes: Synthesis, Solvent-Controlled Cage Structures, and Catalytic Activity

Convenient self-assembly synthesis of copper(II) complexes via double (phenylsilsesquioxane and acetate) ligation allows to isolate a family of impressive sandwich-like cage compounds. An intriguing feature of these complexes is the difference in the structure of a pair of silsesquioxane ligands despite identical (Cu6) nuclearity and number (four) of acetate fragments. Formation of particular combination of silsesquioxane ligands (cyclic/cyclic vs condensed/condensed vs cyclic/condensed) was found to be dependent on the synthesis/crystallization media. A combination of Si4-cyclic and Si6-condensed silsesquioxane ligands is a brand new feature of cage metallasilsesquioxanes. A representative Cu6-complex (4) (with cyclic silsesquioxanes) exhibited high catalytic activity in the oxidation of alkanes and alcohols with peroxides. Maximum yield of the products of cyclohexane oxidation attained 30 %. The compound 4 was also tested as catalyst in the Baeyer-Villiger oxidation of cyclohexanone by m-chloroperoxybenzoic acid: maximum yields of 88 % and 100 % of ϵ-caprolactone were achieved upon conventional heating at 50 °C for 4 h and MW irradiation at 70 or 80 °C during 30 min, respectively. It was also possible to obtain the lactone (up to 16 % yield) directly from the cyclohexane via a tandem oxidation/Baeyer-Villiger oxidation reaction using the same oxidant.

Cage-like Cu5Cs4-Phenylsilsesquioxanes: Synthesis, Supramolecular Structures, and Catalytic Activity

A small family of nonanuclear Cu5Cs4-based phenylsilsesquioxanes 1-2 were prepared by a convenient self-assembly approach and characterized by X-ray diffraction studies. The compounds 1 and 2 show some unprecedented structural features such as the presence of a [Ph14Si14O28]14- silsesquioxane ligand and a CuII5CsI4 nuclearity in which the metal cations occupy unusual positions within the cluster. Copper ions are "wrapped" into a silsesquioxane matrix, while cesium ions are located in external positions. This resulted in cesium-involved aggregation of coordination polymer structures. Both compounds 1 and 2 realize specific metallocene (cesium-phenyl) linkage between neighboring cages. Compound 2 is evaluated as a catalyst in the Baeyer-Villiger (B-V) oxidation of cyclohexanone and tandem cyclohexane oxidation/B-V oxidation of cyclohexanone with m-chloroperoxybenzoic acid (mCPBA) as an oxidant, in an aqueous acetonitrile medium, and HNO3 as the promoter. A quantitative yield of ε-caprolactone was achieved under conventional heating at 50 °C for 4 h or MW irradiation for 30 min (for cyclohexanone as substrate); 17 and 19% yields of lactone upon MW irradiation at 80 °C for 30 min and heating at 50 °C for 4 h, respectively (for cyclohexane as a substrate), were achieved. Complex 2 was evaluated as a catalyst for the oxidation of alkanes to alkyl hydroperoxides and alcohols to ketones with peroxides at 60 °C in acetonitrile. The maximum yield of cyclohexane oxidation products was 30%. Complex 2 exhibits high activity in the oxidation of alcohols.

Homogeneous oxidation of C–H bonds with m-CPBA catalysed by a Co/Fe system: mechanistic insights from the point of view of the oxidant

A Co/Fe system efficiently catalyses the oxidation of C–H bonds with m-CPBA. The nitric acid promoter hampers the m-CPBA homolysis, suppressing the free radical activity. Experimental and computational data evidence a concerted oxidation mechanism.

A novel o-vanillin Fe(III) complex catalytically active in C-H oxidation: exploring the magnetic exchange interactions and spectroscopic properties with different DFT functionals

The novel complex [Fe^IIICl(L)₂(H₂O)] (1) was synthesized by interaction of iron(III) chloride with ethanol solution of o-vanillin (HL) and characterized by IR, UV/Vis spectroscopy, thermogravimetry and single crystal X-ray diffraction analysis. The molecules of 1 in the solid state are joined into supramolecular dimeric units, where a set of strong hydrogen bonds predefines the structure of the dimer according to the "key-lock" principle. From the Hirshfield surface analysis the contribution of π⋯π stacking to the overall stabilization of the dimer was found to be negligible. Broken symmetry DFT calculations suggested the presence of long-range antiferromagnetic interactions (J = -0.12 cm^-1 for H = -JS₁S₂ formalism) occurring through the Fe-O⋯O-Fe pathway, as evidenced by the studies of the model dimers where the water molecules were substituted by acetonitrile and acetone ones. The benchmark studies using a set of literature examples and various DFT functionals revealed the hybrid-GGA B3LYP as the best one for prediction of Fe^III⋯Fe^III antiferromagnetic exchange couplings of small magnitude. Magnetic susceptibility measurements confirmed antiferromagnetic coupling between the metal atoms in 1 with a coupling constant of -0.35 cm^-1. Catalytic studies demonstrated that 1 acts as an efficient catalyst in the oxidation of cyclohexane with hydrogen peroxide in the presence of nitric acid promoter and under mild conditions (yield up to 37% based on the substrate), while tert-butylhydroperoxide (TBHP) and m-chloroperoxybenzoic acid (m-CPBA) as oxidants exhibit less efficiency. Combined UV/TDDFT studies evidence the structural rearrangement of 1 in acetonitrile with the formation of [Fe^IIICl(L)₂(CH₃CN)] species. The TDDFT benchmark using nine common DFT functionals and two model compounds (o-vanillin and [Fe^III(H₂O)₆]³⁺ ion) support the hybrid meta-GGA M06-2X functional as the one most correctly predicting the excited state structure for the Fe(III) complexes, under the conditions studied.

Catalytic Oxidations with Meta-Chloroperoxybenzoic Acid (m-CPBA) and Mono- and Polynuclear Complexes of Nickel: A Mechanistic Outlook

Selective catalytic functionalization of organic substrates using peroxides as terminal oxidants remains a challenge in modern chemistry. The high complexity of interactions between metal catalysts and organic peroxide compounds complicates the targeted construction of efficient catalytic systems. Among the members of the peroxide family, m-chloroperoxybenzoic acid (m-CPBA) exhibits quite complex behavior, where numerous reactive species could be formed upon reaction with a metal complex catalyst. Although m-CPBA finds plenty of applications in fine organic synthesis and catalysis, the factors that discriminate its decomposition routes under catalytic conditions are still poorly understood. The present review covers the advances in catalytic C–H oxidation and olefine epoxidation with m-CPBA catalyzed by mono- and polynuclear complexes of nickel, a cheap and abundant first-row transition metal. The reaction mechanisms are critically discussed, with special attention to the O–O bond splitting route. Selectivity parameters using recognized model hydrocarbon substrates are summarized and important factors that could improve further catalytic studies are outlined.

Phenoxazinone synthase-like catalytic activity of novel mono- and tetranuclear copper(ii) complexes with 2-benzylaminoethanol

Three novel coordination compounds, [Cu(ca)2(Hbae)2] (1), [Cu(va)2(Hbae)2] (2) and [Cu4(va)4(bae)4]·H2O (3), have been prepared by self-assembly reactions of copper(ii) chloride (1 and 2) or tetrafluoroborate (3) and CH3OH (1 and 3) or CH3CN (2) solution of 2-benzylaminoethanol (Hbae) and cinnamic (Hca, 1) or valeric (Hva, 2 and 3) acid. Crystallographic analysis revealed that both 1 and 2 have mononuclear crystal structures, wherein the complex molecules are H-bonded forming extended supramolecular chains. The tetranuclear structure of 3 is based on the {Cu4(μ3-O)4} core, wherein the metal atoms are bound together by μ3 oxygen bridges from 2-benzylaminoethanol forming an overall cubane-like configuration. The strong hydrogen bonding in 1-3 leads to the joining of the neighbouring molecules into 1D chains. Concentration-dependent ESI-MS studies disclosed the equilibria between di-, tri- and tetranuclear species in solutions of 1-3. All three compounds act as catalysts for the aerobic oxidation of o-aminophenol to the phenoxazinone chromophore (phenoxazinone synthase-like activity), with the maximum reaction rates of 4.0 × 10-7, 2.5 × 10-7 and 2.1 × 10-7 M s-1 for 1, 2 and 3, respectively, supported by the quantitative yield of the product after 24 h. The dependence of the reaction rates on catalyst concentrations is evidence of reaction orders higher than one relative to the catalyst. Kinetic and ESI-MS data allowed us to assume that the tetranuclear species, originating from 1, 2 and 3 in solution, possess considerably higher activity than the species of lower nuclearity. Mechanistic and isotopic 18O-labelling experiments suggested that o-aminophenol coordinates to CuII species with the formation of reactive intermediates, while the oxygen from 18O2 is not incorporated into the phenoxazinone chromophore.

A Comparative Study of the Catalytic Behaviour of Alkoxy-1,3,5-Triazapentadiene Copper(II) Complexes in Cyclohexane Oxidation

The mononuclear copper complexes [Cu{NH=C(OR)NC(OR)=NH}2] with alkoxy-1,3,5-triazapentadiene ligands that have different substituents (R = Me (1), Et (2), nPr (3), iPr (4), CH2CH2OCH3 (5)) were prepared, characterized (including the single crystal X-ray analysis of 3) and studied as catalysts in the mild oxidation of alkanes with H2O2 as an oxidant, pyridine as a promoting agent and cyclohexane as a main model substrate. The complex 4 showed the highest activity with a yield of products up to 18.5% and turnover frequency (TOF) up to 41 h−1. Cyclohexyl hydroperoxide was the main reaction product in all cases. Selectivity parameters in the oxidation of substituted cyclohexanes and adamantane disclosed a dominant free radical reaction mechanism with hydroxyl radicals as C–H-attacking species. The main overoxidation product was 6-hydroxyhexanoic acid, suggesting the presence of a secondary reaction mechanism of a different type. All complexes undergo gradual alteration of their structures in acetonitrile solutions to produce catalytically-active intermediates, as evidenced by UV/Vis spectroscopy and kinetic studies. Complex 4, having tertiary C–H bonds in its iPr substituents, showed the fastest alteration rate, which can be significantly suppressed by using the CD3CN solvent instead of CH3CN one. The observed process was associated to an autocatalytic oxidation of the alkoxy-1,3,5-triazapentadiene ligand. The deuterated complex 4-d32 was prepared and showed higher stability under the same conditions. The complexes 1 and 4 showed different reactivity in the formation of H218O from 18O2 in acetonitrile solutions.

Carbene insertion to N–H bonds of 2-aminothiazole and 2-amino-1,3,4-thiadiazole derivatives catalyzed by iron phthalocyanine

Iron(III) phthalocyaninate decorated with crown ether substituents, [(15C5)4PcFe]Cl, efficiently catalyzed the insertion of carbene derived from ethyl diazoacetate to six amines functionalized with thiazole, thiazoline and thiadiazole heterocycles. The reactions were carried out under practical conditions using EDA:amine stoechiometric ratio with 0.05 mol% catalyst loading. Turnover numbers up to 3360 have been achieved. The aminoacid derivatives bearing heterocyclic moieties were obtained under catalytic conditions for the first time with 36–69% yields in the case of single N–H insertion products and up to 77% in the case of double N–H insertion products.

Tetracopper(II) Cores Driven by an Unexplored Trifunctional Aminoalcohol Sulfonic Acid for Mild Catalytic C–H Functionalization of Alkanes

Three new tetracopper(II) coordination compounds were easily generated from Cu(NO3)2, a trifunctional aminoalcohol sulfonic acid (H3bes, N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid) as a principal building block, and a benzene carboxylic acid as a supporting ligand (i.e., benzoic (Hba), 4-hydroxybenzoic (Hfba), or 3-hydroxybenzoic (Hthba) acid). The obtained microcrystalline products, [Cu4(µ-Hbes)3(µ-H2bes)(µ-L)]·2H2O (L = ba− (1), fhba− (2), and thba− (3)), were fully characterized by FTIR (Fourier-transform infrared spectroscopy), elemental analysis, ESI-MS (Electrospray Ionisation Mass Spectrometry), and single-crystal X-ray diffraction methods. Compounds 1–3 were applied as effective homogeneous catalysts in the oxidative C−H functionalization of alkanes (cycloalkanes and propane). Two different model reactions were explored: (1) mild oxidation of alkanes with hydrogen peroxide to give alcohols and ketones, and (2) mild carboxylation of alkanes with carbon monoxide, water, and potassium peroxodisulfate to give carboxylic acids. For these reactions, effects of different parameters, as well as mechanistic and selectivity characteristics, were studied.

Heterometallic CoIIIZnII Schiff Base Catalyst for Mild Hydroxylation of C(sp3)–H Bonds of Unactivated Alkanes: Evidence for Dual Mechanism Controlled by the Promoter

The novel Schiff base complex [CoIIIZnIIL3Cl2]·CH3OH (1) was synthesized by interaction of zinc powder, cobalt(II) chloride and methanol solution of the pre-formed HL in air (HL is the product of condensation of o-vanillin and methylamine) and characterized by IR, UV-Vis and NMR spectroscopy, ESI-MS and single crystal X-ray diffraction analysis. In the heterometallic core of 1 the two metal centers are bridged by deprotonated phenoxy groups of the L− ligands with the cobalt-zinc separation of 3.123 Å. Catalytic investigations demonstrated a pronounced activity of 1 towards mild alkane oxidation with m-chloroperbenzoic acid (m-CPBA) as an oxidant and cis-1,2-dimethylcyclohexane (cis-1,2-DMCH) as the model substrate. The influence of the nature of different promoting agents of various acidities (from HOTf to pyridine) on the catalytic process was studied in detail and a pronounced activity of 1 in the presence of nitric acid promoter was found, also showing a high retention of stereoconfiguration of the substrate (>99% for cis-1,2-DMCH). The best achieved yield of tertiary cis-alcohol based on the oxidant was 61%, with a turnover number (TON) of 198 for nitric acid as promoter. The 18O-incorporations into the alcohols when the reactions were performed under 18O2 atmosphere using acetic and nitric acid promoters, suggest that the cis-1,2-DMCH hydroxylation proceeds by two distinct pathways, a non-stereoselective and a stereoselective one (with and without involvement of a long-lived free carbon radical, respectively). The former dominates in the case of acetic acid promoter and the latter is realized in the case of HNO3 promoter.